1. Field of the Invention
The present invention relates to an optical element, a liquid crystal cell and liquid crystal display device with the optical element and a method of producing the optical element.
2. Description of the Related Art
Liquid crystal display devices (LCD), having advantages such as being capable of easily thinning and saving weight, capable of saving consumption power and difficult to generate flickers, are used in various fields such as TVs and medical equipments.
A liquid crystal display device is provided with a liquid crystal cell that controls retardation of light and the liquid crystal cell is sandwiched between polarization plates in a thickness direction of the liquid crystal cell to switch light (control of an amount of light propagating toward an observer side of a liquid crystal screen on which an image is displayed). The liquid crystal cell has two substrates that face each other with a definite distance separated and a liquid crystal layer (driving liquid crystal layer) formed by encapsulating a liquid crystal material between two substrates is constituted by forming so that the alignment properties of liquid crystal molecules (driving liquid crystal molecules) contained in the liquid crystal material may be controlled by an electric field. Regarding two substrates, a substrate near to an observer of a liquid crystal screen is named a display side substrate and a substrate that faces the display side liquid crystal substrate with a driving liquid crystal layer sandwiched therebetween is named a driving liquid crystal side substrate. A liquid crystal display device is provided with, as needs arise, outside of a polarization plate, a backlight unit that supplies light propagating through a liquid crystal layer in a liquid crystal cell in a direction from the driving liquid crystal side substrate to the display side substrate.
A liquid crystal screen provided to a liquid crystal display device is formed by gathering a lot of pixels and an image displayed on the liquid crystal screen is formed of lights propagating outward from the respective pixels (toward an observer). In order to improve quality of an image displayed on a liquid crystal display device, it is important to strictly control light propagating toward an observer, that is, it is important to adjust retardation of light transmitting a driving liquid crystal layer formed in a liquid crystal cell thereby strictly controlling light switching.
In such a liquid crystal display device, in some cases, a thickness of the driving liquid crystal layer may vary unexpectedly largely owing to an acting force externally applied on a liquid crystal cell during production or usage of a liquid crystal cell. When a liquid crystal display device undergoes an unexpected large change in a thickness of a driving liquid crystal layer, an amount of retardation of light, which shows the optical characteristics thereof, unexpectedly varies. This makes switching control of light difficult, and it directly results in deteriorating image quality displayed on a liquid crystal screen.
In this connection, usually, in order to inhibit a thickness of a driving liquid crystal layer from readily varying owing to an external acting force, a lot of hard spacers are dispersively arranged between two opposed substrates. However, when a layer structure largely softer than the spacers is formed on a substrate that constitutes a liquid crystal cell, even when spacers arranged in a liquid crystal display device are very hard, ultimately, there remains a problem that a thickness of a driving liquid crystal layer readily varies. That is, for example, when, in a liquid crystal display device, a soft layer structure is formed on a substrate surface and on the layer structure spacers are dispersively arranged, owing to an external acting force, the spacers readily subside in the layer structure to result in changing a thickness of the driving liquid crystal layer.
However, recently, an optical element where a liquid crystal material containing liquid crystal molecules (polymerizable liquid crystal molecules) having a polymerizable functional group is coated on a substrate to form a coating film and the polymerizable liquid crystal molecules contained in the coating film are polymerized to make the coating film a birefringent layer as an optical functional layer having an optical compensation function that causes birefringence of light is proposed and a liquid crystal display device where an optical element is incorporated so that the birefringent layer may be located between a substrate and a driving liquid crystal layer is proposed (for example, Japanese Patent Application Laid Open (JP-A) No. 2001-100045 and Japanese Patent Application National Publication (Laid Open) No. 10-508882). In the case of the liquid crystal display device, a situation that, on a birefringent layer that is an optical functional layer formed on a substrate, spacers are dispersively arranged to constitute a liquid crystal display device happens. In the liquid crystal display device constituted like this, in comparison with a liquid crystal display device in which a birefringent layer is not formed, the possibility that the spacers such as mentioned above subside tends to be larger; accordingly, a situation that a thickness of a driving liquid crystal layer easily unexpectedly varies is likely to occur.
Regarding the point, in JP-A No. 2006-28346, as to an optical element having an optical functional layer obtained by polymerizing polymerizable liquid crystal molecules, in order to keep the hardness of the optical functional layer high, a technology where, in particular, the hardness of the optical functional layer is set to 17 or more by Vickers hardness or B or more by the pencil hardness is proposed.
However, the Vickers hardness or pencil hardness such as shown in JP-A No. 2006-28346 cannot be said a value obtained by a measurement method that sufficiently reflects actual conditions of the spacers generally used in recent liquid crystal display devices and actual conditions of general usage environment of liquid crystal cells.
For example, the Vickers hardness value in JP-A No. 2006-28346 is, as shown in JIS Z 2244-03, a value obtained by measuring an indentation depth of an optical functional layer by use of an indenter with a quadrilateral tip end. Accordingly, the obtained value cannot be said a value that is measured by a measurement method that reflects actual conditions of the spacers, where, in recent years, cylindrically-shaped spacers are usually used for a liquid crystal display device.
Furthermore, the pencil hardness value in JP-A No. 2006-28346 is a value obtained by measuring the scratch hardness of an optical functional layer; accordingly it cannot be said a value measured by a measurement method that sufficiently reflects actual conditions that spacers subside relative to the optical functional layer to be capable of varying a thickness of a driving liquid crystal layer.
From above-mentioned reasons, an optical element that, by sufficiently reflecting actual conditions of a liquid crystal display device, can effectively reduce fear that a thickness of a driving liquid crystal layer unexpectedly varies and thereby can maintain the thickness substantially constant and a liquid crystal display device where the optical element is incorporated are in demand.